Automatic shut height motor control circuit



April 3, 1962 F. E. HEIBERGER ETAL 3,028,531

AUTOMATIC SHUT HEIGHT MOTOR CONTROL CIRCUIT Filed July 29, 1957 55 H ma u m w Vsw r 0 W85 T Z J: Z A .m T #k K4 3 L Q 5 m W MAIN M m %MY F0 PM W, 0 a 7 2 MB 0, m 2 :.M i M m M a M 0 1 Mn M 4 M 6 2 11 6 mm 1 1 L M waifw I I I |.l M a fi 4 m m w United States Patent 3,028,531 AUTOMATIC SHUT HEIGHT MOTOR CONTROL CIRCUIT Francis E. Heiberger, Elmlrnrst, and Daniel T. Kennedy,

La Grange Park, 11]., assignors to Dauly Machine Specialties, Inc., Chicago, 111., a corporation of Illinois Filed July 29, 1957, Ser. No. 674,800 16 Claims. (Cl. 318-29) Our invention relates to an automatic shut height control circuit for a power press and more particularly to a circuit by means of which the shut height of a press may be adjusted in a rapid, convenient and expeditious manner.

The shut height of a press by definition is the distance between the press slide and the bolster with the slide drive in its bottom dead center position. Maximum shut height, of course, is the distance between the slide and bolster with the slide adjustment in its extreme raised position. Each time a power press is set up the shut height must be adjusted to a critical distance. Accomplished by the usual trial and error method, this adjustment takes a considerable amount of time not only during the initial set up of the die but also when the die is returned to production after storage or after a regrinding operation.

In the early days of the power press art a change in shut height could only be accomplished byusing filler plates. Later hand crank operated slide adjustments were provided. Most present day power presses are equipped with motor driven, slide adjusting mechanisms. The rate of production of modern presses is so great that die changes occur frequently and may even occur from one work shift to the next on large presses. On smaller presses production runs as low as two hours are common. With the motor-driven slide adjustments of mod ern power presses where a large change in shut height is necessary, the slide adjustment push buttons on the press must be held depressed for a period of as long as five minutes for a ten-inch change in shut height. If the entire length ofadjustment is to be traversed in the course of a shut height adjustment as long as eighteen minutes may be consumed, since normal slide adjustment drives move at a rate of two inches a minute or less. Even after the approximate adjusted position has been reached it is necessary to jog the adjustment motor, measure shut height, jog again, try a stamping, adjust, and so forth. It will be appreciated that although the motor-driven adjusting means has reduced the physical burden to the operator a shut height adjustment remains a tedious and time-consuming operation. appreciated that it is desirable that a shut height adjustment be accomplished rapidly with a high degree of accuracy.

We have invented an automatic shut height control circuit for a power press which accomplishes a shut height adjustment in a rapid, convenient and expeditious manner. Our circuit provides a very accurate shut height adjustment.

One object of our invention is to provide an automatic shut height control circuit for a power press which performs a shut height adjustment in a rapid, convenient and expeditious manner.

Another object of our invention is to provide an automatic shut height control circuit for rapidly adjusting the shut height of a power press with a high degree of accuracy.

Other and further objects of our invention will appear from the following description.

In general our invention contemplates the provision of an automatic shut height control circuit for a power press including manually operable means for producing a first. voltage proportional to the desired shut height. A second From the foregoing it will be ICC , error voltage representing the difference between actual shut height position and the desired shut height position. First means responsive to the error signal energizes the slide adjustment drive motor to move the slide on the press frame at a slow speed. Second means responsive to the error signal energizes the adjustment drive motor to operate at a higher speed when, the error signal exceeds a predetermined value. Our circuit includes means re sponsive to the motion of the slide for reducing the error signal to zero as the slide approaches the desired position. We provide our circuit with auxiliary means for generating an error signal of a predetermined value to move the slide away from the bolster through a predetermined distance to permit operations to be performed on the dies. 1

In the accompanying drawing which forms part of the instant specification and which is to be read in conjunction therewith:

The single FIGURE is a schematic view of one form of our automatic shut height control circuit for a power press.

Referring now to the drawing, our control circuit in-- means driven by motor 10 form no part of our invention,

we have not shown these elements in detail in the drawing. One example of such a slide-adjusting means is shown in the copending application of Philo H. Danly and Vasil Georgefr, Serial No. 516,934, filed June 21, 1955, now Patent No. 2,923,263 for a reciprocating bed metal drawing press. V

Our control circuit includes a Wheatstone bridge, indicated generally by the reference character 16, having a pair of input terminals 18 and 20 across which we con- ,nect a full wave rectifier bridge, indicated generally by the reference character 22, having a smoothing capacitor 24 connected across its output terminals. We connect the primary winding 26 of a transformer, indicated generally by the reference character 28, to the terminals 30 and 32 of a suitable source of alternating current potential. We connect the primary winding 34 of a transformer, indicated generally by the reference character 36, across the secondary winding 38 of transformer 28. The secondary winding 40 of transformer 36 is connected across the input terminals of rectifier 22. From the circuitry thus far described it will be seen that rectifier 22 provides a full wave rectified input voltage for the bridge 16.

We connect terminal 18 to the brush 42 of a potentiometer winding 44 connected in series with respective potentiometer windings 46, 48 and 50 in our bridge. The terminal 20 of bridge 16 forms a common connection between windings 48 and 50. Our bridge includes respective output terminals 52 and 54. A normally open relay switch 1SAU1 and a normally closed relay switch 1SAD1 connect the bridge output terminal 52 to respective brushes 56 and 58 associated with windings 46 and 48. We connect output terminal 54 to the brush 60 associated with potentiometer winding 50. A dial knob 62 carried by the press frame (not shown) is manually operated to drive a linkage 64 which moves the brushes 56 and 58 in unison to corresponding positions on their associated windings 46 and 48. As is known in the art motor 10, when energized, drives the press slide-adjusting means to move the slide (not shown) on the press frame. The slide in turn drives the brush 60 of po tentiometer winding 5G to position the brush in accordance with the actual adjusted position of the slide on the press. We have indicated this connection between rnotor 1'0 and brush 6t) schematically in the figure by the linkage 66.

I From the foregoing description it will be appreciated that the portion of winding 48 between brush 58 and terminal 20 of thebridge 16 carries a voltage which is proportional to the position to which it is desired to adjust the press slide in adjusting shut height. The portion of winding 50 between brush .60 and terminal 20 carries a voltage which is proportional to the actual adjusted position of the press slide on the frame and hence proportional to the actual shut height. The potentiometer including winding 44 provides means for compensating fordifierences in resistance in the conductors con necting the various bridge elements. It will be apparent that the output terminals 52 and 54 of the bridge carry an error signal which is proportional to the difference between the actual adjusted position of. the slide andthe position to which it is desired to move the slide.

We connect respective output conductors 68 and 711 to the terminals 5-2 and 54. Our circuit includes a p-n-p transistor 72 having a base 74, an emitter 76 and a collector 78. A conductor 81 connects the emitter 76 to conductor 68. Arectifier 82 and a clipper diode 84 connect the base 74 to conductor 70. We connect the primary winding 86 of a transformer, indicated generally by the reference character 88, across the secondary winding 38 of transformer 28. We connect a full wave rectifier, indicated generally by the reference character 90, across the secondary winding 92 of transformer 83. A capacitor -.92 connected across the output terminals of rectifier 90 smooths the rectified voltage. We connect the winding 94 of a potentiometer across capacitor 92. We connect a relay winding 3SAU between one terminal of winding 94 and the collector 78 and connect the other terminal of the potentiometer winding 94 to conductor.80. A resistor.96 connected between the brush 98 and the base 74 of transistor 72 provides a means for biasingthe transistor to determine thepoint at which thetransistor stops conducting in a manner to be described.

Assuming that bridge .16 is initially. balanced so that the voltage appearing between the output terminals '52 and 54is substantially zero, if knob 62 is turned to move brush 58 in a counterclockwise direction, .as viewedfin the figure, output terminal 52 becomes positive with respect to terminal 54. As a result of this operation emitter 76 is made positive with respect to base 74- to cause a current iiow through the transistor from conductor 80 through the transistor through clipper diode 84, which prevents high currents from flowing through the transistor, and through the diode 82 which protects the transistor from high reverse voltages. At the same time the potential across potentiometer winding 94 causes a current flowin the collector circuit of the transistor to energize winding 3SAU. It is to be understood that the application of the potential picked off potentiometer Winding- 94 and applied to the base and emitter of transistor 72 permits a predetermined current flow through relay winding 3SAU even in the absence of an error signal.- This current flow is not, however, suflicient to pick up relay 3SAU. When an error signal is generated in the manner described, it is applied to emitter 76 to permit additional current flow through winding 3SAU to cause the winding to be energized. In other words, before an error signal occurs, winding 3SAU is conditioned to be energized upon the occurrence of a very small error signal.

This portion of our control circuit may, for example, cause an upward movement of the press slide as will be explained hereinafter under the action of the low speed drive for the slide. This up control-portion of our circuit includes a second transistor 160' having an emitter 102 connected to conductor 68, a base 104 and a collector 196. We connect a clipper diode 16S and a current-limiting resistor 110 in series between base 1-64 and the common terminal of diodes 82 and 84. We connect a relay winding ZSAU in series with a resistor 112 normally shunted by a normally closed switch SSAUZ 7 between collector 106 and the terminal of winding 94 to which winding SSAU is connected. If the unbalance in bridge "16 resulting from a movement of brush 58 is great enough current will flow through transistor from conductor 68,-through the transistor, through clipper diode 108, through current-limiting resistor 114 a and through diode 82 to conductor '70.

We connect a normally open switch 2SAU1 in series 10 in series with a rheostat122 having a resistance-adjusting brush 124. The full wave'rectifier 1 26 also supplies a voltage-dividingresistor 12-8 with a potential for energizing the armature 12 of motor 10. We connect the 'brush associated with register 128 to one terminal 132 of the motor armature circuit. Respective normally open switches 2SA1 and 3SA1 are adapted to be closed i to short out the brush 130 to apply the full potential provided by rectifier 126 tothe motor armature. A conductor 134 connects the terminal of resistor 138 remote fromthe normally open switches ZSA-Land 3SA1 to the other terminal 136 of the motor armature circuit. We'

connect a normally open switch 4SA1 and a normally closed switch 1SA 1 between terminal 132 and one of the terminals of armature 12, We connect a normally open switch 1SA2 between terminal 132 and theother terminal of armature 12. A normally open switch 1SA3, and a normally closed-switch 4SA3jare adapted to connect the terminal 136 to the armature terminal to which.

switch 4SA1'is connected. A normally open switch 4SA2 is adapted to be closed to connect terminal .136

to the terminal of armature 12 to which switch 1SA2 is connected.

Let us assume that knob 62 has been operated to cause terminal 52 to become positive with respect to terminal 54 to cause transistor 72 to conduct to energize winding SSAU. Energization of this winding closes switch SSAUI to energize windings 48A and 4ASA. The energization of winding 4SA closes switch 4SA1 andswitch 4SA2 and opens switch 4SA3. As a result ofthis operation a circuit is complete from brush 130 through switches 1SA1 and 4SA1 through armature 12 and through switch 4SA2 to terminal 136. With this energization of armature 12 motor 10, rotates to drive the press slide and linkage 66 to move brush 60 in a clockwise direction as viewed in the figure to reduce the bridge output voltage to zero. I

As has been explained hereinabove, closing of switch 3SAU1 energizes winding 4ASA. We connect anormally open switch 4ASA1 associated with winding 4ASA in series with a signal lamp 138 between conductors 114 and 116. We connect a normally'open switch 3SAD1, a normally closed switch 4ASA2 and a relay winding ISA in series between conductors 114 and 116. A conductor 149 connects the common terminal of switches 3SAD11 and 4ASA2 to the common terminal of switch 4ASA1 and lamp 138. Energization of winding 4ASA opens switch 4ASA2 and closes switch 4ASA1 to cause signal lamp 138 to light to indicate that an adjustment of the slide is taking place.

As has been explained hereinabove, dial 62 is adjusted to cause a large'unbalance in bridge 16 transistor We also coning 2SAU. Energization of winding ZSAU closes the normally open switch 2SAU1 to energize winding 35A to cause the normally open switch 3SA-1 to close. switch 3SA1 closes, the full potential of rectifier 126 isapplied to the armature 12 to cause motor to drive the slide at high speed. This high speed drive continues until the output potential of bridge 16 drops below a point at which it causes such current to flow through transsistor 100 to maintain winding ZSAU energized. Energization of 'winding SSA also opens switch 3SA2 to cause resistor 112 to be placed in the collector circuit of transistor 100 to limit current flow in this circuit. When the relay winding ZSAU is deenergized switch 3SA1 opens to reduce the potential applied to armature '12 to the potential appearing between brush 130 and conductor 134.

We also provide our circuit with means for driving the slide adjusting means to move the slide down when adjustment of knob 62 causes terminal 52 to become negative with respect to terminal 54. We connect the emitter 142 of a transistor 144 to conductor 70. A iode 146 which protects transistor 144 from high reverse potentials and a clipper diode 148 connect the base 150 of transistor 144 to conductor 68. We connect the primary winding 152 of a transformer, indicated generally by the reference character 154, across the secondary winding 92. A full wave rectifier, indicated generally by the reference character 156, connected across the secondary winding 158 of transformer 154- rectifies the transformer output voltage. We connect the winding 160 of a potentiometer having a brush 162 across a smoothing capacitor 164 connected across the output terminals of rectifier 156. We connect a relay winding SSAD between one terminal of winding 160 and the collector 166 of transistor 144 and connect the other terminal of winding 160 to the emitter 142. it will be seen that rectifier 156 provides the collector supply voltage for transistor 144. A resistor 164 connected between brush 162 and base 150 provides a means for biasing this transistor to determine the point at which'the transistor stops conducting to interrupt the circuit of motor 18 as the bridge 16 comes to balance.

From the foregoing it will be seen that if terminal 52 is negative with respect to terminal 54 emitter 142 is positive with respect to base 158 to cause a current flow through the transistor from conductor 78 through the transistor 144 through clipper diode 148 and through diode 146 to conductor 68. At the same time current flows in the collector circuit of the transistor to energize winding 3SAD. As will be explained hereinafter, transistor 144 is the slow speed control element for a down movement, for example, of the slide.

The down adjusting portion of our circuit includes a transistor 168 having an emitter 170 connected to conductor 70, a collector 1-72 and a base 174. A clipper diode 176 and a current limiting resistor 178 connect base 174 to the common terminal of diodes 146 and 148. We connect a relay winding 2SAD and a resistor 180 which is normally shunted by a normally closed switch 2SA2 in series between the terminal of winding 16%) to which winding SSAD is connected and the collector 172 of transistor 168..

Ashas been explained hereinabove, we connect a switch 3SAD1 associated with winding 3SAD in series with switch 4ASA2 and winding ISA between conductors 114 and116. We connect a normally open switch 2SAD1 in series with a winding 2SA between conductors 114 and 116. Assuming-that terminal 54 is positive with respect to terminal 52 to cause transistor 144 to conduct to energize winding 3SAD, this winding closes switch 3SAD1 to energize winding ISA through switch 4ASA2 and to cause signal lamp 138 to light. Energization of winding ISA opens normally closed switch 1SA1 and closes normally a open switches 1SA2 and 1SA3 In this manner a circuit is complete from brush 130 through switch 1SA2, through When a to be made in the die area of the press.

normally open push button switch 182 in series with a 6 armature 12 and through switches 1SA3 and 4SA3 to conductor 134. Completion of this circuit energizes the armature to cause motor 18 to drive the press slide and linkage 66 to move brush 60 in a direction to restore the bridge to balance. It is to be noted that in this condition of our circuit a current flows through armature 12 in a direction opposite to the direction of current flow under the action of transistors 72 and 100.

If the amount of bridge unbalance is sufliciently great transistor 168 conducts to energize winding 2SAD to cause switch 2SAD1 to close to energize winding 2SA. Energiz-ation of this winding 2SA closes switch 2SA1 to bypass brush to place the full potential of rectifier 126 across armature 12 to cause motor 10 to drive at high speed. Energization of winding 2SA also opens switch 2SA2 to place resistor in the collector circuit of transistor 168. As the-bridge 16 returns to balance under the action of the high speed drive the bridge output potentialdrops below the point at which it sustains sufficient current flow through transistor 168 to maintain winding 2SAD energized. When this relay winding 2SAD is de-energized switch 2SA1 opens to reduce the potential applied to armature 12 to cause the motor to drive at slow speed.

' It is desirable that the press slide be adapted to be rapidly movedaway from the bolster (to permit repairs We connect a relay winding 1SAU between conductors 114 and 116. We connect a signal lamp 184 in parallel with winding ISAU. A normally closed push button switch 186, which is ganged with switch 182, is connected in series with a relay winding ISAD between conductors 114 and 116. As is shown in the drawing switch 186 is normally closed to energize winding 1SAD .to maintain switch 1SAD1 closed. At the same time relay winding 1SAU is not energized so that switch 1SAU1 is open. In this condition our circuit functions normally to adjust the slide to the desired position indicated by the position of knob 62. As has been explained hereinabove, brushes 56 and 58 occupy corresponding positions on their associated windings 46 and 48 with the result that a potential of a predetermined magnitude exists between the brushes.

and 58 to cause terminal 52 to become positive with respect to terminal 54. As has been explained hereinabove,'

such an unbalance causes motor 10 to move the slide up. When the repairs have been accomplished the button of switches 182 and 186 is released again to energize winding 1SAD and to de-energize winding ISAU to permit the circuit to return the slide to a position corresponding to the position of knob 62. It will be appreciated that lamp 184 lights when switch 182 is closed to indicate that the slide is being moved up.

In operation of our automatic shut height control circuit when it is desired, for example, to increase shut height, knob 62 is turned tomove brushes 56 and 58 in a counterclockwise direction as viewed in the figure to transistors conduct as the bridge returns to balance tran-. sistor 100 first stops conducting to reduce the speed of motor 10. When the bridge is very nearly balanced transistor 72 stops conducting to interrupt the circuit of the 'to manual operation of a dial. tediousand time-consuming adjusting techniques used in the prior art. Further, our circuit permits the slide to be moved away from the bolster rapidly to permit minor sulting potential difference is sufficiently great both transistors 144 and 168 conduct With the result that motor initially drives the press slide down at high speed. As the slide approaches the desired position transistor 168 stops conducting to reduce the motor speed. As the slide arrives at the desired position transistor 144 stops conducting to cause the motor ltito stop. The point at which the low speed transistor 144 stops conducting may readily be adjusted by moving brush 162 along winding 160.

If minor repairs are to be accomplished in the die area 1 of the press we push the button o-f switches 182 and 186 to energize winding 1SAU and to dc-energize winding ISAD to cause switch 1SAU1 to close and to cause switch iSADI to open. As has been explained hereinabove this action so unbalances bridge16 as to cause the slide to be moved rapidly upwardly through a distance proportional to thepotential difference between brushes 56 and 58. When the repairs, have been accomplished the button of switches 182 and 186'is released and the slide returns automatically to the position from which it was moved. v H Our automatic shut height control circuit must have a high sensitivity if the actual shut height is to be made correspondent to the desired shut height within very close limits. Accordingly, to obtain this high sensitivity, the bridge voltage atinput terminals 1 8 and supplied I responsive to said slide adjusting means for producing" by the secondary winding 40 of transformer 36 is made very high. Further, the bridge output error voltage at terminals 52 and 54 appearing on conductors 68 and 70 is amplified by our transistor circuit. Finally, transistors '72 and 144 are biased by adjusting the voltage appearing at potentiometer brushes 98 and 162 such that, with zero bridge output voltage, the current flowing through relay windings AU and SSAD, through collectoljs 78 and 166 respectively, is almost but not quite 'suflicient to actuate these relays. By thus biasing transistors 72 and 144 to the threshold current of these relay windings, only a small bridge output voltage is required to actuate either relay 35AU or 35AD according as the polarity of the output error voltage. The high bridge input voltage at terminals 18 and 20, although greatly increasing the sensitivity of our transistor control circuit,

f creates the problem of preventing the characteristics or" the transistors from being adversely afiected by the large error signal produced especially when contact 1SAD1 opens and contact 1SAU]. closes to cause a predetermined bridge unbalance." Crystals 82 and 146 prevent high reverse voltages from being applied between the emitters and bases of the transistors. Crystals 84, 108, 148, and 176 prevent excessive forward currents in the base circuits of the transistors; and hence these diodes diodes should preferably have the characteristic of permitting appreciable reverse current to flow before the in- V cremental back resistance becomes very high. This backto-back connection offcrystal 82 with each of crystals 84 and 108 and of crystal 146 with each of crystals 148 and 176 protects our transistor amplifier from the large bridge output voltages which may be occasioned because of the high sensitivity afforded by the highbridge input voltage.

It will beseen that we have accomplished the objects of our invention. We have provided an automatic shut height adjusting control circuit. Our circuit automatically adjusts shut height to a desired position in response Our circuit avoids the repairs in the die area and automatically returns the slide to the position from which it was moved to accomplish the repairs. e It will be understood that certain features and'sub combinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of our claims. It is further obvious that various changes may be made in details within the scope of our claims without departing from the spirit of our invention. It is, therefore, to

be understood that our invention is not to be limited to the specific details shown and described.

Having thus described our invention, what we claim is: 1. In a power press having a bed and a slide carried by a frame and means for adjusting the position of the slide with respect to the bed a shut height adjusting circuit including in combination a two-speed drive means for said slide-adjusting means, means for producing a first signal proportional to the desired shut height, means a second signal proportional to the actual shut height, means for comparing said signals to produce an error signal, means responsive to an error signal of a first magnitude for energizing said drive means to drive said ally operable n'ieans for varying the relative resistances of said second pair of arms.

3. A circuit as in claim 1 in which said signal-ptoducing and comparing means comprises a bridge having an output terminal and including a pair of resistance windings having respective brushes associated therewith, means for gauging said brushes, a normally closed switch and a normally open switch for connecting the respective,

brushes to said output terminal and manually operable means for closing said normally open switch and for opening said normally closed switch to cause a predetermined electrical unbalance in said bridge.

4. A circuit as in claim 1 in which said drive means is a motor having an armature and in which said means responsive to said error signalscomprise means for applying a signal of one polarity to said armature in response to an error signal of one polarity and means for applying a signal of the other polarity to said armature in response to an error .signal of said other polarity.

5. A circuit as in claim 1 in which said means responsive to an error signal of a first magnitude, cornprises a transistor and means for biasing said transistor to conduct in respons to an error signal of said first magnitude.

6. A circuit as in claim 1 in which said means responsive to an error signal of said second magnitude comprises a transistor and a current-limiting resistor connected-in the base circuit of said transistor.

7. A circuit as in claim 1 in which said'means responsive to said first magnitude signal comprises a first transistorand means for biasing said transistor to conduct in response toan error signal of said first magnitude and in which said means'responsive to an error signal of the second magnitude comprises a second transistor and means for biasing said second transistor to conduct in transistor, means for biasing said transistor to conduct in response to said first magnitude error signal and means for adjusting said biasing means to regulate the point at which said transistorstops conducting.

9. In a power press having a frame'and a bed and a slide carried by the frame and means for adjusting the position "of the slide relative to the bed, it "shut height adjusting circuit including in combination drive means for the slide adjusting means, a bridge circuit having an output terminal and including a pair of resistance windings having respective brushes associated therewith, means for ganging the brushes to move in synchronism, a normally closed switch, a normally open switch, means including the normally closed switch for connecting one of the brushes to the bridge output terminal, means including the normally closed switch for connecting the other of the pair of brushes associated with the pair of resistance windings to the output terminal, means for simultaneously closing the normally open switch and opening the normally closed switch to cause a predetermined unbalance signal at the bridge output terminal, and means responsive to the unbalance signal for energizing the drive means.

10. In a power press having a bed and a slide mounted for reciprocating movement on a frame and having means adapted to be driven to adjust the position of said slide with respect to said bed shut height adjusting apparatus including in combination means'adaptcd' to be energized to drive said slide adjusting means, means for producing a first signal proportional to the desired shut height of said slide over said bed, means responsive tov said slide adjusting means for producing a second signal proportional to the actual shut height, means for com' paring said first and second signals to produce an error signal representing the difference between the actual shut height and the desired shut height, means comprising relay means adapted to be energized in response to said error signal for actuating said drive means to drive said slide adjusting means in a direction to reduce said difference to zero and means responsive to energization of said relay means for reducing the sensitvity of said relay means.

ll. In a power press having a bed and a slide mounted for reciprocating movement on a frame and having means adapted to be driven to adjust the position of said slide with respect to said bed shut height adjusting apparatus including in combination means adapted to be actuated to drive said slide adjusting means, means for producing a first signalproportional to the desired shut height of said slide over said bed, means responsive to said slide adjusting means for producing a second signal proportional to the actual shut height, means for comparing said first and second signals to produce an error signal representing the distance between the actual shut height and the desired shut height, means responsive to said error signal for actuating said drive means to drive said slide adjusting means to reduce said difference to zero, means adapted to be actuated to alter said first signal to produce an error signal for actuating said drive means to drive said slide adjusting means to move said slide away from said bed to a predetermined position and means for actuating said signal altering means to move said slide to said predetermined position to permit access to the space between said slide and said bed.

12. In a power press having a bed and a slide mounted for reciprocating movement on a frame and having means adapted to be driven to adjust the position of said slide with respect to said bed shut height adjusting apparatus including in combination means adapted to be actuated to drive said slide adjusting means, a bridge circuit having a first pair of arms and a second pair of arms, means for varying the relative impedances of the first pair of arms to obtain a first signal representative of the desired shut height, means responsive to said slide adjusting means for varying the relative impedances of the second pair of arms to obtain a second signal representative of the actual shut height whereby the bridge normally produces an output error signal representative of the diiference between the actual shut height and the desired shut height, means responsive to the error signal for actuating said drive means normally to drive the slide adjusting means to reduce the error signal to zero and means adapted to be actuated to shift a predetermined portion of the impedance of one arm of said first pair to the second arm of said first pair to produce a predetermined unbalance in said bridge circuit to energize said drive means to drive said slide adjusting means to move said slide to a positionover said bed which is a predetermined distance above said desired shut height.

13; In a power press having a bed and a slide mounted for reciprocating movement on a frame and having means adapted to be driven to adjust the position of said slide with respect to said bed shut height adjusting apparatus including in combination means adapted to be energized to drive said slide adjusting means, means for producing a first signal proportional to the desired shut height of said slide over said bed, means responsive to said. slide adjusting means for producing a second signal proportional to the actual shut height, means for comparing 'said first and second signals to produce an error signal representing the difference between the actual shut height and the desired shut height, means responsive to the error signal existing when the difference between the actual shut height and the desired shut height is greater than a certain distance for actuating said drive means to drive said slide adjustingmeans at a first predetermined rate in a direction to reduce said difference, means responsive to the error signal existing when the difference between the actual shut height and the desired shut height is within said certain distance for actuating said drive means to drive said slide adjusting means at a sec ond rate which is slower than said first predetermined rate in a direction to reduce said difference to zero, each of said error signal responsive means comprising a unidirectional amplifier responsive to an error signal of one polarity for producing. a current in a given direction to actuate said drive means and a diode connected in series with said amplifier for limiting the flow of current in said given direction.

14. In a power press having a bed and a slide mounted for reciprocating movement on a frame and having means adapted to be driven to adjust the position of said slide with respect to said bed shut height adjusting apparatus including in combination means adapted to be actuated pair of arms to obtain a second signal representative of the actual shut height whereby the bridge normally produces an output error signal representative of the difference between the actual shut height and the desired shut height, means responsive to the error signal for actuating said drive means normally to drive the slide adjusting means to reduce the error signal to zero, means adapted to be actuated to shift a predetermined portion of the impedance of one arm of said first pair to the second arm of said first pair to produce a predetermined unbalance in said bridge circuit to energize said drive means to drive said slide adjusting means to move said slide to a position over said bed which is a predetermined distance above said desired shut height, said error signal responsive means comprising a unidirectional amplifier responsive to an error signal of one polarity for producing a current in a given direction to energize said drive means and a diode connected in series with said amplifier for limiting the current flow in said given direction.

15. In a power press having a bed and a slide mounted for reciprocating movement on a frame and having means adapted to be driven to adjust the position of said slide with respect to said bed shut height adjusting apparatus including in combination means adapted to be energized to drive said slide adjusting means, means for producing a first signal proportional to the desired shut height of said slide over said bed, means responsive to said slide adjusting means for producing a second signal proportional tothe actual shut height, means for comparing said first and second signals to produce an error signal representing the difference between the actual shut height and the desired shut height, means responsive to the error signal existing when the difference between the factual shut height and the desired shut height is greater mined rate in a direction to reduce said difierence, means responsive to the error signal existing when the differ;

ence between the actual shut height and the desired shut height is within said certain distance for actuating said drive means to drive said slide adjusting means at a second rate which is slower than said first predetermined rate in a direction to reduce said difference to zero, means adapted to: be actuated to alter said first signal to produce an error, signal for actuating said drive means to drive said slide adjusting means to move said slide away from said bed to a predetermined location, means for actuating said signal altering meansto permit access to the space between the slide and the bed, each of said error signal responsive means comprising a unidirectional amplifier responsive to a signal of one polarity for producing a current in a given direction to energize said drive means to drive said slide adjusting means and a duce an error signal for actuating said drive means to 16. a In a power press having a bed and aslide mounted for reciprocating movement on a frame and having means adapted to be driven to adjust the position of said slide with respect to said bed shut height adjusting apparatus including in combination means adapted to be energized to drive said slide adjusting means, means for produc ing a first signal proportional to the desired shut height of said slide over said bed, means responsive to said slide adjusting means for producing a second signal proportional to the actual shut height, means for comparing said first and second signals to produce an error signal representing the difference between the actual shut height and the desired shut height, means responsive to the error signal existing when the difference between the actual shut height and the desired shut height is greater than a certain distance for actuating said drive means to drive'said slide adjusting means at a first predeterdiode connected in series with said amplifier for limiting the current flow in said given direction.

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